1. Field of the Invention
The present invention relates to the construction and location of inductive links for implanted electrical devices, particularly but not exclusively cochlear prostheses.
2. Description of the Prior Art
A transdermal inductive link is commonly used to communicate power and data to implanted devices such as multi-channel cochlear implants. An internal coil is implanted with the device, and an external coil is positioned adjacent to it externally. In cochlear prostheses, it is common to locate the inductive components on the side of the head adjacent the mastoid bone, above and behind the pinna or outer ear. Other positions on the body have also been used, for locating transdermal links for instance on the chest. The inductive coils used are circular or open loops, and typically have a diameter of several centimeters. The external coil is connected to the speech processor via a cable.
In U.S. Pat. No. 4,932,405 there is described a cochlear implant system in which power is transmitted from an external processor and transmitter to an implanted receiver/stimulator by means of an inductive coil inserted into the ear canal. However, the disclosed system requires that data communication between the external and internal devices be carried out by means of a separate, preferably infra-red, communication channel which makes the transcutaneous crossing at a point some distance from the coil. Furthermore the inductive coil used for receiving the transmitted energy is to be implanted around the auditory duct which is a far from straightforward procedure.
In a paper, "Energizing Implantable Transmitters by means of Coupled Inductance Coils", Kadefors et al, p177-183 177, Vol BME-16, No 3, (July 1969) of IEEE Transactions on Bio-Medical Engineering, the authors discuss the use of ferrite cores to increase the efficiency of inductive transfer to implanted devices however the paper is concerned with only the transmission of power to an implanted circuit not with the more complex problem of supplying both power and data.
In recent years, there has been a trend to miniaturise the speech processor to the extent that it can be mounted behind the pinna. This miniaturisation has however not extended to the coil, which is still separate from the speech processor, connected to it by a cable, and usually mounted some distance above and behind the pinna.
The separate connection of the speech processor to the coil by a cable gives rise to a number of practical difficulties. It is not aesthetically pleasing, and the resulting effect on their appearance is of considerable concern to patients. A further difficulty is that the connecting cable is prone to breakage, and is accordingly a source of unreliability within the system.
An object of this invention is to provide an inductive arrangement of reduced size, so as to enable the elimination of the separate coil and processor arrangement and the cable that connects them.
A further object of the invention is to provide an inductive arrangement of reduced size which can facilitate transmission of both power and data.